1. Field of the Invention
The present invention relates to a magnetic recording medium which has a magnetic film suitable for high-density magnetic recording.
2. Description of the Related Art
Magnetic disk devices so far put in practical use employ a longitudinal magnetic recording system. The longitudinal magnetic recording system is a recording system wherein a longitudinal domain parallel to a disk substrate is formed on a magnetic recording medium which can be easily magnetized in a direction parallel to the plane of the substrate, and the recording density of the longitudinal magnetic recording is being increased. An increase in the recording density is achieved mainly by improving a coercive force and decreasing the thickness of a magnetic film.
For the purpose of improving the coercive force of a magnetic film of Co alloy, there is employed a method for providing such a material having a body-centered cubic (bcc) structure as Cr or Cr alloy or providing an underlayer having a B2 type crystal structure such as NiAl between a magnetic film and a substrate (see David E. Laughlin. Y. C. Feng. David N. Lambeth, Li-Lien Lee, Li Tang, xe2x80x9cDesign and crystallography of multilayered mediaxe2x80x9d in Journal of Magnetism and Magnetic Materials, Vol. 155, pp. 146-150, 1996).
When an underlayer of the bcc structure is employed, it is effective in further improving the coercive force to match lattice constant conditions of a Co alloy magnetic film to be epitaxially grown with those of the underlayer. (For example, refer to a U.S. literature (N. Inaba, A. Nakamura, T. Yamamoto, Y. Hosoe, M. Futamoto, xe2x80x9cMagnetic and crystallographic properties of CoCrPt thin films formed on Crxe2x80x94Ti single crystalline underlayersxe2x80x9d in Journal of Applied Physics, Vol. 79, No. 8, pp. 5354-5356, 1996).
For improving the coercive force, it is further effective to improve the crystallographic quality of the Co alloy magnetic film. To this end, it is effective to add a new underlayer of nonmagnetic material of a hexagonal close-packed (hcp) structure (which will be referred to merely as the hcp structure, hereinafter) having the same crystalline structure as the Co alloy magnetic film between the underlayer and magnetic film (for example, refer to JP-A-4-321919). Usable as the underlayer to be added for a Coxe2x80x94Crxe2x80x94Pt magnetic film as one of the typical magnetic films is, for example, a Co film containing 35 atomic % of Cr, that is, a Co-34 at. %Cr (for example, refer to M. Futamoto, Y. Honda, Y. Hirayama, K. Itoh, H. Ide, Y. Maruyama, xe2x80x9cHigh Density Magnetic Recording on Highly Oriented CoCr-Alloy Perpendicular Rigid Disk Mediaxe2x80x9d in IEEE Transactions on Magnetics, Vol. 32, No. 5, pp. 3789-3794, 1996).
In view of these literatures, the recording density of the longitudinal magnetic recording medium reaches currently 10 Gb/in2. In order to obtain a recording density exceeding 10 Gb/in2, it becomes necessary to additionally increase the coercive force and also to enhance the thermal stability characteristics. The improvement of the recording density requires the decrease of the thickness of the magnetic film, as mentioned above. However, when the film thickness is too small, this involves such a problem that thermal influence results in decrease of recording magnetization with time, leading in some cases to disappearance of the magnetization. The influence by such thermal stability of the magnetization becomes remarkable when the magnetic film is a Co alloy-based magnetic film often used in the magnetic disk device and when the thickness of the magnetic film is usually 20 nm or less.
Based on the conventional method for improving a recording density, it is considered that, in order to realize a recording density of 10 Gb/in2 or more, the magnetic film has to have a thickness of 20 nm or less and thus its thermal influence cannot be avoided.
It is therefore an object of the present invention to provide a novel magnetic recording medium which can realize a high recording density of 10 Gb/in2 or more and can be put in practical use, by increasing a coercive force and improving thermal stability characteristics.
As a result of having examined thermal stability characteristics of a longitudinal magnetic recording medium, it has been found that a region having a deteriorated crystallographic quality with a low magnetic anisotropy energy is formed on a part of a magnetic film, and the deterioration of the crystallographic quality expands from the region to promote decrease of the recording magnetization. Further, the deteriorated crystallographic quality part has a low magnetic anisotropy energy and the magnetic anisotropy energy is used to form the coercive force of the medium, so that existence of a region having a deteriorated crystallographic quality results in reduction of the medium coercive force, that is, decrease in the recording magnetization. Accordingly it is important to block formation of such a deteriorated crystallographic quality region.
From the aforementioned viewpoint, when Co alloy material most generally used in a longitudinal magnetic recording medium and having the hcp structure in discussion is examined, it has been found that a region having a low crystallographic quality is present in an initial growth region of a magnetic film, and the crystallographic quality of the region is strongly affected by an underlayer contacted with the magnetic film. Therefore, it is clearly important to set the structure and composition of the underlayer in such a manner as to provide a good crystallographic quality for the magnetic film.
As a result of having paid attention to and examined a magnetic-film formation process, it has also been found that, when at least one very thin layer having a composition different from that of the magnetic film is introduced in the magnetic-film formation process, the magnetic anisotropy energy of crystalline grains forming the magnetic film can be increased.
The present invention is based on the aforementioned consideration and examination results. In accordance with an aspect of the present invention, the above object is attained by providing a magnetic recording medium featured in that an underlayer has a two-layer structure of a lower underlayer contacted with a substrate and an upper underlayer contacted with a Co alloy magnetic film, the upper underlayer is a Coxe2x80x94Crxxe2x80x94My alloy film having a hexagonal close-packed (hcp) structure, where an addition concentration of Cr+nonmagnetic element M satisfies a relationship of 25 atomic %xe2x89xa6x+yxe2x89xa650 atomic %, an addition amount of the element M satisfies a relationship of 0.5 atomic %xe2x89xa6y, and the element M is one selected from the group of elements B, Si, Ge, C, Al, P, Ti, V, Nb, Zr, Hf, Mn, Rh, Os, Ir, Re, Pd, Pt, Mo, Ta, W, Ag and Au. In addition, the upper underlayer has feature such that the alloy film has the hcp structure of Coxe2x80x94Ruxxe2x80x94Cry (5 atomic % less than xxe2x89xa665 atomic %, 35 atomic %xe2x89xa7yxe2x89xa79 atomic %).
Further, in accordance with another aspect of the present invention, there is provided a magnetic recording medium featured in that a Co alloy magnetic film is separated by at least one very thin film of Coxe2x80x94Crxxe2x80x94My alloy having a hexagonal close-packed structure, an addition concentration of Cr+M satisfies a relationship of 25 atomic %xe2x89xa6x+yxe2x89xa650 atomic %, an addition amount of M satisfies a relationship of 0.5 atomic %xe2x89xa6y, nonmagnetic element M is one selected from the group of elements B, Si, Ge, C, Al, P, Ti, V, Nb, Zr, Hf, Mn, Rh, Os, Ir, Re, Pd, Pt, Mo, Ta, W, Ag and Au.
In longitudinal magnetic recording, the material of the magnetic film of media most often used and studied so far is Co alloy having a hexagonal close-packed (hcp) structure. In a Co alloy-based longitudinal magnetic recording medium, a region having a low crystallographic quality is present in an initial film growth region. The crystallographic quality of this part is strongly affected by its underlayer. The present invention is featured in that the underlayer has a structure of at least two layers, and an upper underlayer contacted with the magnetic film is made of a selected material.